In order to filter oil in engines and transmissions, a suction oil filter apparatus or a pressure oil filter apparatus can be used. In the case of a suction oil filter apparatus, a pump is arranged at the oil outlet of the apparatus, with the pump sucking the oil to be filtered through a filter medium by means of negative pressure. When seen in the direction of flow, the pump of the filter apparatus is thus downstream. It is common practice to arrange a suction oil filter apparatus directly in an oil sump of an oil pan, with the entire oil to be filtered flowing through said suction oil filter apparatus. Such a construction requires only little additional space and is thus compact and does not require any additional connecting lines. When designing such an apparatus, there will always be a balance between the maximum permitted differential pressure between oil inlet and oil outlet of the apparatus and a filtering performance resulting therefrom. Since a minimum oil volume flow needs to pass through the suction oil filter even in the case of high viscosity of the oil or at low temperature of the oil in order to thus achieve sufficient lubrication of an engine or a transmission, only a relatively low filtering performance is achieved. A suction filter apparatus can thus be evaluated in a simplified manner as a coarse filter.
In the case of a pressure oil filter apparatus, the oil to be filtered is pressed by means of a pump into a filter apparatus. Such an apparatus is usually arranged in a bypass and can be activated by means of a valve for example. A pressure oil filter apparatus is often arranged within the transmission or on the transmission housing, with hose connections or pipe connections being required outside of the transmission. In comparison with a suction oil filter apparatus such a construction requires a relatively large amount of space, with the weight and the costs of a transmission or engine increasing substantially by the connecting lines. Since only a part of the oil volume flow to be filtered, which is mostly less than 50%, flows through such a pressure oil filter apparatus, a highly efficient filter medium in pleated form for example can be used. A relatively high amount of soiling with dirt is linked to such a high filter performance, so that a filter medium of a pressure oil filter apparatus needs to be exchanged relatively frequently. The pressure oil filter apparatus can be destroyed in the case of excessive oil pressure, so that overpressure protection needs to be integrated in the pressure oil filter circulation by means of a bypass valve for example or an orifice plate.
In order to achieve the highest possible oil purity within a short period of time, a combination of suction oil filter apparatus through which passes the entire oil volume flow and a pressure oil filter apparatus through which passes only a bypass flow of 20 to 40% of the entire oil volume flow can be provided. The disadvantageous aspect is the constructional and instrumental complexity of peripheral parts such as adapters, housing, oil guidance, etc. for the pressure oil filter apparatus, including the required additional space. As a result of the high pressure load, a highly stable and heavy construction of the apparatus is required. The costs for such a combination of suction oil filter apparatus and pressure oil filter apparatus are thus relatively high.